Thermal management of advanced gas turbine engines is one of the most difficult constraints on continued performance improvements. Improved thermodynamic efficiency drives the cycle designer to higher compressor pressure ratios, which necessarily result in increased compressor discharge temperatures (above, for example, 700° C.). Higher performance is also obtained by continually increasing the gas temperature entering the turbine. Because the compressor discharge air is also used to cool the high pressure turbine nozzles and blades, the combination of increased turbine inlet temperature and cooling air temperature places extreme demands on the material capability of these components.
One method for alleviating the thermal load on the turbine can be accomplished by extracting turbine cooling air from the main flowpath and reducing its temperature. Due to the unacceptable drag load of an air/air heat exchanger, the only acceptable sink for the excess heat is the fuel stream. Two major difficulties with this approach are as follows: (1) the fuel has a limited capacity to absorb heat due to thermal degradation and coking/fouling of the heat exchanger; and (2) a fuel/air heat exchanger will have high stress levels due to the large thermal differential across the two fluid streams, and will likely suffer from low cycle fatigue cracking. The fuel pressure is always higher than the air pressure, and thus any crack will permit fuel to enter the air side of the heat exchanger. At typical operating temperatures, auto-ignition of the fuel leak and a catastrophic failure of the heat exchanger and turbine are therefore highly probable.
Given the above, there is a need in the art for systems that are designed to address various thermal management issues in advanced gas turbine engines.